making a 7/16 -10 acme left hand tap,making a 7/16 -10 acme left hand tap, version 3 by gerry...
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Gerry Goldberg/R.G. Sparber Page 1 of 21 August 22, 2011
Making a 7/16 -10 Acme Left Hand Tap, version 3
By Gerry Goldberg as told to R. G. Sparber
Pictures by Neil Butterfield
Copyleft protects this article1.
Background
On 8/13/2011 the Valley Metal Club of Phoenix convened at the shop of Gerry
Goldberg. Gerry is a master machinist with skills in many areas. On this particular
day, he taught us how to make a tap.
1 You are feel to copy and distribute this article but not change it.
Gerry Goldberg/R.G. Sparber Page 2 of 21 August 22, 2011
Before we arrived, Gerry had turned a tap blank from O-1 tool steel. When we
arrived, he had the blank between centers and had ground a HSS cutter to the
profile of an Acme thread. Dimensions can be found in the Machinery’s
Handbook®.
This O-1 tool steel is rather tough so Gerry started by taking 0.003” per pass until
the cutter started to load up. Then he went to 0.002” per pass. After a few passes he
took only 0.001” per pass. It was slow going but you can see that he is getting
excellent results. He is using Tap Magic® as his cutting fluid.
Gerry pointed out that this same cutter can be used to make worm gears.
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He is making the tap because one of his lathe’s cross feed nut is worn out. He has
pulled the lead screw and is using the 3 wire method to measure its thread size. The
tap will be cut to match it.
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Here he is applying the same 3 wires to the tap and will mic it.
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When the thread is finally done, it is time to cut the two flutes. Ideally he would
support the end of the tap with a jack but did not have one handy. So instead he
will be taking 0.01” deep cuts for a total depth of 0.1”.
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The 3/16” ball end mill has been carefully centered on the longitudinal axis of the
tap and is ready to cut the first flute.
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Here is the first flute. The top edges are very sharp.
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The second flute is now being cut.
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Here is the tap fully machined. Gerry next ground a bright spot on the square shark
to make it easier to see color changes during heat treating.
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A can full of old oil is ready to
quench the tap. Gerry is using
an Oxy-Acetylene torch to
evenly heat the tap. Care must
be taken not to heat the part so
much that those sharp edges
burn off. If sparks are seen, tiny
bits of metal are being lost.
The edge can be restored by
grinding if necessary.
The tap is heated to cherry red
but only around the threads. The
shank is left un-hardened.
Towards the end of the heating,
Gerry put the tap on some fire
bricks.
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When the proper temperature was reached, the tap was plunged into the oil. If fully
submerged, there will be minimal flame. You can buy special quenching oil that
does not flame much.
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The tap is too hot to handle but has now been fully quenched and is ready for
grinding.
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First Gerry runs it under the tap for final cooling.
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Using a diamond coated wheel, back relief is ground into the end of the tap. This
could be done with a surface grinder but free hand is fine.
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The cutting edge is right at the flute and the perimeter behind it has been ground
away a little to provide the necessary back relief.
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The tap is now very hard and can snap if stressed. So to give the tap back a little
flexibility, Gerry will use a MAPP gas torch to bring the tap up to straw color
which is around 400° F. We all watched as Gerry got it to the right color. I for one
could not tell when it was straw color. All I know is that if it turns blue, you went
too far and must start over with hardening.
Once the proper temperature is reached, Gerry quenched the tap in oil to stop the
annealing process. Then the tap was again cooled in water.
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The final step was to use a diamond coated wheel to cut some relief inside each of
the flutes.
Gerry Goldberg/R.G. Sparber Page
The second flute’s relief has not been cut yet. This is an end view
cutting point. Remember, this is a left hand tap.
Gerry Goldberg/R.G. Sparber Page 18 of 21 August 22
has not been cut yet. This is an end view looking into the
Remember, this is a left hand tap.
August 22, 2011
looking into the
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Here is the first trial of the tap going through brass. It is a left hand tap so it is easy
to forget and turn it the wrong way.
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As a final test, the leadscrew is threaded through the hole.
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Well, the next time I need some crazy size tap, I will certainly try to make it
myself.
Rick Sparber (ghost writing for Gerry).